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WASHINGTON (CNN) -- Two teams of government investigators using fake documents were able to enter the United States with enough radioactive sources to make two dirty bombs, according to a federal report made available Monday.

The investigators purchased a "small quantity" of radioactive materials from a commercial source, according to a Government Accountability Office report prepared for Senate Permanent Subcommittee on Investigations Chairman Norm Coleman, a Minnesota Republican.

"It's just an indictment of the system that it's easier to get radiological material than it is to get cold medicine," said a senior subcommittee staffer about the findings.

The report, along with two others by the GAO on the subject of smuggling and detection of nuclear materials, were provided to reporters by congressional sources in advance of the first of two hearings by the subcommittee scheduled to begin Tuesday.

The focus will be on what the federal government has done to protect the country against nuclear terrorism. This week's hearings come after almost three years of bipartisan and bicameral investigations into the subject.

A second GAO report notes that while the departments of State, Energy and Defense have provided radiation-detection equipment to 36 countries since 1994 to combat nuclear smuggling, operating the equipment has proven challenging.

Those challenges include technical limitations of some of the equipment, a lack of supporting infrastructure at some border sites and corruption of some foreign border security officials.

The report also notes that the State Department, the lead interagency coordinator in this effort, has not maintained a master list of U.S.-funded radiation-detection equipment in foreign countries.

Without such a list, program managers at the agencies involved "cannot accurately assess if equipment is operational and being used as intended; determine the equipment needs of countries where they plan to provide assistance; or detect if an agency has unknowingly supplied duplicative equipment," the report says.

It further criticizes the State Department, saying that "without taking steps to ensure that all previously provided radiation-detection equipment, specifically hand-held equipment, is adequately maintained and remains operational, State cannot ensure the continued effectiveness or long-term sustainability of this equipment."

A third GAO report observes that, while the Department of Homeland Security has made progress in deploying radiation-detection equipment at U.S. ports -- which include 670 portal monitors and more than 19,000 pieces of hand-held radiation detection equipment as of last December -- the agency's program goals are "unrealistic" and its cost estimate is "uncertain."

GAO's analysis concluded that the program may exceed its budget by $342 million.

David McIntyre, a spokesman for the Nuclear Regulatory Commission, told CNN that the agency disagreed with the GAO over the risk significance of the materials taken across the border, but then said he didn't know what materials were involved.

The NRC ranks radioactive materials by order of their security significance, such as radioactivity, dispersability and how attractive they might be to terrorists.

On the issue of the fake NRC documents downloaded from the Internet and doctored by the GAO investigators to get their shipment past border officials, McIntyre said, "We are concerned about their ability to counterfeit an NRC document, and we are taking steps to address that."

The steps include finding ways to make NRC documents more difficult to counterfeit and working with customs officials if they need information about NRC licenses or licensees.